AP_Frsky_Telem: replace VDOP with extra GPS status bits

This removes VDOP (not really that useful) and replaces two of those
bits with GPS status info that now allows to differentiate between 3D
fix, DGPS, RTK Float, and RTK Fixed. This is written to maximize
backwards compatibility (by not shifting any other bits of the
gps_status variable)

libraries/AP_Frsky_Telem/AP_Frsky_Telem.cpp

@@ -633,12 +633,12 @@ uint32_t AP_Frsky_Telem::calc_gps_status(void)
 
     // number of GPS satellites visible (limit to 15 (0xF) since the value is stored on 4 bits)
     gps_status = (_ahrs.get_gps().num_sats() < GPS_SATS_LIMIT) ? _ahrs.get_gps().num_sats() : GPS_SATS_LIMIT;
-    // GPS receiver status (limit to 3 (0x3) since the value is stored on 2 bits: NO_GPS = 0, NO_FIX = 1, GPS_OK_FIX_2D = 2, GPS_OK_FIX_3D or GPS_OK_FIX_3D_DGPS or GPS_OK_FIX_3D_RTK = 3)
+    // GPS receiver status (limit to 0-3 (0x3) since the value is stored on 2 bits: NO_GPS = 0, NO_FIX = 1, GPS_OK_FIX_2D = 2, GPS_OK_FIX_3D or GPS_OK_FIX_3D_DGPS or GPS_OK_FIX_3D_RTK_FLOAT or GPS_OK_FIX_3D_RTK_FIXED = 3)
     gps_status |= ((_ahrs.get_gps().status() < GPS_STATUS_LIMIT) ? _ahrs.get_gps().status() : GPS_STATUS_LIMIT)<<GPS_STATUS_OFFSET;
     // GPS horizontal dilution of precision in dm
     gps_status |= prep_number(roundf(_ahrs.get_gps().get_hdop() * 0.1f),2,1)<<GPS_HDOP_OFFSET; 
-    // GPS vertical dilution of precision in dm
-    gps_status |= prep_number(roundf(_ahrs.get_gps().get_vdop() * 0.1f),2,1)<<GPS_VDOP_OFFSET; 
+    // GPS receiver advanced status (0: no advanced fix, 1: GPS_OK_FIX_3D_DGPS, 2: GPS_OK_FIX_3D_RTK_FLOAT, 3: GPS_OK_FIX_3D_RTK_FIXED)
+    gps_status |= ((_ahrs.get_gps().status() > GPS_STATUS_LIMIT) ? _ahrs.get_gps().status()-GPS_STATUS_LIMIT : 0)<<GPS_ADVSTATUS_OFFSET;
     // Altitude MSL in dm
     const Location &loc = _ahrs.get_gps().location();
     gps_status |= prep_number(roundf(loc.alt * 0.1f),2,2)<<GPS_ALTMSL_OFFSET; 

libraries/AP_Frsky_Telem/AP_Frsky_Telem.h

@@ -76,7 +76,7 @@ for FrSky SPort Passthrough
 #define GPS_STATUS_LIMIT            0x3
 #define GPS_STATUS_OFFSET           4
 #define GPS_HDOP_OFFSET             6
-#define GPS_VDOP_OFFSET             14
+#define GPS_ADVSTATUS_OFFSET        14
 #define GPS_ALTMSL_OFFSET           22
 // for battery data
 #define BATT_VOLTAGE_LIMIT          0x1FF